[C++ PATCH] Fix ICE on empty statement expression (PR c++/79822)

[Jakub Jelinek]

Hi!

Seems callers of cxx_eval_constant_expression don't really like if it
returns NULL, which is what cxx_eval_statement_list returns for empty
STATEMENT_LIST.  In statement expressions we treat those the same
as (void) 0 or any other statement with void value.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2017-03-03  Jakub Jelinek  <ja...@redhat.com>

        PR c++/79822
        * constexpr.c (cxx_eval_statement_list): Treat empty ({ }) like
        ({ (void) 0; }).

        * g++.dg/cpp0x/constexpr-79822.C: New test.

--- gcc/cp/constexpr.c.jj       2017-02-22 23:03:12.000000000 +0100
+++ gcc/cp/constexpr.c  2017-03-03 12:42:02.391773591 +0100
@@ -3725,8 +3725,9 @@ cxx_eval_statement_list (const constexpr
 {
   tree_stmt_iterator i;
   tree local_target;
-  /* In a statement-expression we want to return the last value.  */
-  tree r = NULL_TREE;
+  /* In a statement-expression we want to return the last value.
+     For empty statement expression return void_node.  */
+  tree r = void_node;
   if (!jump_target)
     {
       local_target = NULL_TREE;
--- gcc/testsuite/g++.dg/cpp0x/constexpr-79822.C.jj     2017-03-03 
12:49:41.595614494 +0100
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-79822.C        2017-03-03 
12:50:51.753673495 +0100
@@ -0,0 +1,12 @@
+// PR c++/79822
+// { dg-do compile }
+// { dg-options "" }
+
+bool
+foo ()
+{
+  bool a = ({ }) && false;             // { dg-error "could not convert" }
+  bool b = ({ ; }) && false;           // { dg-error "could not convert" }
+  bool c = ({ (void) 1; }) && false;   // { dg-error "could not convert" }
+  return a && b && c;
+}

        Jakub